1. Field of the Invention
The present invention relates generally to direct-type back light devices, and more particularly, to a direct-type back light device, which is capable of maintaining uniform luminance, and reducing manufacturing costs and power consumption, in addition to being easy to assemble and to accomplish thinness.
2. Description of the Related Art
A liquid crystal display (LCD) is an electric element which converts several electric signals indicative of information produced from various units into visual signals using a transmittance of liquid crystal which is changed in response to an applied voltage, prior to transmitting visual information to a person. Since the LCD itself cannot emit light to produce images, the LCD must be provided with a back light device. The images are produced on the LCD by light radiated from the back light device.
There is a growing tendency for a back light device which has high and uniform luminance, and is low in power consumption and manufacturing costs thereof, and has a thin structure due to a compact arrangement of parts.
FIG. 1 is a perspective view of a conventional direct-type back light device, when a diffusion member is separated from a frame of the back light device. FIG. 2 is a view to show an arrangement of cold cathode fluorescent lamps and inverters included in the direct-type back light device of FIG. 1.
As shown in FIGS. 1 and 2, the direct-type back light device includes a frame 10. A diffusion member 20, a plurality of cold cathode fluorescent lamps 30 which will be referred to as a fluorescent lamp hereinafter, and a reflection member 40 are provided in the frame 10 to be superposed on each other. Both ends of each fluorescent lamp 30 are connected in series, to a capacitor 51 which is provided on an output end of each of inverters 50. Thus, when alternating voltage is applied to each of the inverters 50, the applied alternating voltage is boosted in a transformer 52 provided in each of the inverters 50, and then is supplied to each fluorescent lamp 30 which is connected to each of the capacitors 51 in series.
Each fluorescent lamp 30 radiates light by the alternating voltage supplied to the fluorescent lamp 30. The light is reflected by the reflecting member 40. The radiated and reflected light is uniformly distributed over a panel (not shown) of the LCD through the diffusion member 20, thus producing images on the LCD.
However, because the conventional direct-type back light device is constructed so that the plurality of inverters are connected to the plurality of fluorescent lamps in series, the conventional direct-type back light device has a problem as follows.
The conventional direct-type back light device has a problem in that it is impossible to manufacture the inverters connected to the fluorescent lamps in series so that the transformers provided in the inverters have the same boosting ratio, thus there exists difference between the boosting voltages of the inverters. Due to such difference of the boosting voltages, electrostatic capacities of the capacitors are different from each other, thus reducing uniformity of the luminance of the back light device.
The conventional direct-type back light device has another problem in that the number of the inverters is increased as the number of the fluorescent lamps is increased, in case where the inverters are connected to the fluorescent lamps in series, thus increasing manufacturing costs, and increasing power consumption due to the use of a large number of inverters.
The conventional direct-type back light device has a further problem in that a large number of inverters are mounted to a lower surface of the reflection member, so that it is difficult to handle leads which connect the inverters to the fluorescent lamps and thereby it is difficult to assemble the product. Recently, because a large-sized LCD has been developed, the above-mentioned problems become more serious.